Compounds

Introduction

Compounds are pure substances that are made up of two or more elements that are chemically combined in fixed mass ratios. The elements in the compound are joined together by chemical bonds.

The properties of a compound are unique are greatly differ from the elements that make up the compound.

How do you name compounds?

When compounds are named the name of the second element is changed to an “ide” suffix to indicate that the elements are chemically combined and are not just mixtures. Also by convention the metals are named first. Eg. The reaction of magnesium metal and oxygen produces the compound magnesium oxide. The reaction of iron and sulfur produces the compound iron sulfide.

Naming of Binary Compounds

Element 1

Element 2

Compound ("ide" suffix)

Sodium

Chlorine

Sodium chloride

Barium

Sulfur

Barium sulfide

Aluminum

Oxygen

Aluminum oxide

Magnesium

Nitrogen

Magnesium nitride

Potassium

Fluorine

Potassium fluoride

What is a chemical formula?

All compounds have a chemical formula which tell us

the elements that are present in the compound and

the ratio of these elements in the compound.

For example

the chemical formula for water is H2O. This means that there are two parts hydrogen to one part oxygen.

the chemical formula for salt is NaCl. This means that the ratio of sodium to chlorine parts is 1:1

Ionic or covalent?

Compounds may be divided into two main groups, ionic compounds or covalent compounds.

As a general rule it is helpful to know the following

Ionic Compounds

Covalent compounds

Made of a metal + non-metal

Eg. Sodium chloride, NaCl

Made of non-metal + non-metal

Eg. Water, H2O

Composed of positive and negative ions

Eg. Na+ and Cl-

Are good electrolytes

Composed of neutral or unchanged molecules.

Eg. H2O

Are poor electrolytes

An electrolyte is a liquid that conducts an electric current. The liquid may be molten or aqueous which means dissolved in water.

Ionic substances are made up of charged particles called ions. eg. Na+ and Cl-

Positive ions are called cations and negative ions anions. A helpful way of remembering cations are positives is that the t in cations looks like a + sign!

The positive sodium ion, Na+ is an example of a cation and the negative chlorine ion, Cl- is an example of anion.

When an ionic compound is dissolved in water the positive and negative ions are surrounded by the water molecules. The symbol aq means aqueous or surrounded by water molecules.

The dissolving of sodium chloride or common table salt, chemical formula NaCl is given by the following chemical equation.

NaCl(s) → Na+(aq) + Cl-(aq)

(s) = solid

(aq) = aqueous

Ionic substances are good electrolytes or conductors of electricity when dissolved in water because the charged ions are able to move in an electric field.

The covalent compound water on the other hand is a relatively poor conductor of electricity. It is made up mostly of neutral water particles which are not affected by an electric field.

Please note that water always contains a small amount of H+ and OH- ions and is able to conduct electricity at high voltages, so always be careful. Only test the conductivity of water at school under the directions of a science teacher. A low voltage power pack will be used.